Free piston internal-combustion engine



Feb. 8, 1955 R. HUBER FREE PISTON INTERNAL-COMBUSTION ENGINE Filed Oct. 4, 1949 -INVENTUR RT H BER ATTORNEYS United States Patent FREE PISTON INTERNALCOMBUSTION ENGINE Robert Huber, Bellevue, France, assignor to Societe dEtudes & de Participations, Eau, Gaz, Electricite, Energie, S. A., Geneva, Switzerland, a society of Switzerland Application October 4, 1949, Serial No. 119,512 Claims priority, application France November 5, 1948 4 Claims. (Cl. 123-46) The present invention relates to free piston machines. It is more particularly but not exclusively concerned with free piston auto-generators. Free piston auto-generators are machines including two portions, to wit a compressor portion and a motor portion, the air compressed in the compressor portion being fed to the motor portion, which delivers a gaseous mixture of the excess of scavenging air and the incompletely expanded combustion gases. This mixture is used to drive a receiver machine, in particular a gas turbine.

The object of my invention is to provide a machine of this kind which complies better than those known at the present time with the various requirements of practice.

My invention consists chiefly in varying, at least during the period of normal operation of the machine, the final compression pressure of the motor cylinder in accordance with the load of the'free piston machine, in particular in accordance with the working pressure of this machine, in such a manner that the compression in the motor cylinder increases when the load of the machine increases, and vice versa, this variation of the compression in the motor cylinder being obtained through a corresponding variation of the energy which ensures the return movement of the free piston system.

Preferred embodiments of my invention will be hereinafter described with reference to the accompanying drawings, given merely by way of example and in which:

Fig. 1 diagrammatically shows, in axial section, a free piston auto-generator made according to my invention;

Figs. 2 and 3 are diagrams illustrating respectively the variations of the mean pressure of the return energy pneumatic accumulator and the variations of the maximum pressure of compression in the motor cylinder in response to variations of the mean pressure existing in the casing of the auto-generator shown by Fig. 1;

Fig. 4 shows, in axial section, another embodiment of the stabilizer fitted on an auto-generator such as shown by Fig. 1.

Fig. 5 is a partial view showing a modification of the stabilizer of Fig. 4.

Concerning this auto-generator, with the exception of its regulating means, it may be constituted in any suitable manner, for instance as shown by Fig. 1 of the drawing.

Such an auto-generator includes a motor cylinder 1 in which operates a motor piston 2 rigid with a compressor piston 3 operating in a compressor cylinder 4, the two pistons 2-3 constituting a free piston system.

The motor portion of this auto-generator, which includes cylinder 1 and piston 2, works on the two-stroke diesel principle. This is why cylinder 1 is fitted with a fuel injector 5, with inlet ports 6 for the scavenging and feed air and with exhaust ports 7 through which escapes a mixture constituted by the excess of scavenging air and the incompletely expanded combustion gases, this mixture serving to drive, for instance, a gas turbine (not shown by the drawing).

Ports 6 and 7 are controlled by motor piston 2.

Concerning the compressor portion, which includes piston 3 and cylinder 4, the cylinder chamber located on the inner side of piston 3 is fitted with inlet valve means 8 and discharge valve means 9; these last mentioned valve means serve to deliver the air compressed in said inner chamber into a casing 10 which surrounds motor cylinder 1.

The chamber of cylinder 4 located on the outer side of piston 3 acts as a return energy accumulator. The

mass of air present in this last mentioned chamber is compressed during the outward stroke of piston structure 23, that is to say during the working stroke of motor piston 2, in order to store up nearly the whole of the energy developed during this stroke in the motor cylinder. During the next stroke, this air, when expanding, restores this energy to piston 3 which, during this last mentioned stroke, compresses the air present in the inner chamber of cylinder 4, discharges this air into casing 10 and compresses the combustion air present in motor cylinder 1.

It is known that, in a gas turbine plant, when the load of the turbine increases, it is necessary to have both the pressure of the motor gas supplied to said turbine and the amount (weight) of this gas increased.

The amount of gas supplied by the auto-generator depends, on the one hand, upon the length of stroke of the piston system, which length is variable in a free piston machine and, on the other hand, upon the number of oscillations of this system per unit of time.

In order to reduce the variations of the length of stroke when the load of the plant varies, it may be desirable to vary to a considerable degree the number of oscillations per unit of time, in accordance with the load, so as thus to adapt the output of the auto-generator to the various instantaneous values of the load of the plant.

This is why, according to the main feature of my invention, I vary, at least during the period of normal operation of the auto-generator, the final compression in the motor cylinder and, consequently, the maximum pressure of combustion in this cylinder, in accordance with the load of the machine in such a manner that the final compression in the motor cylinder increases when the load increases and vice versa, this variation of the final compression in the motor cylinder being obtained through a corresponding variation of the value of the return energy supplied by the return energy accumulator. This variation of the final compression and of the maximum pressure of combustion in the motor cylinder and the corresponding variation of the return energy have the efiect of varying the number of oscillations of the piston system per unit of time.

Of course, some limits are to be complied with for the variations of the maximum pressure of compression in the motor cylinder. The lower limit is imposed by the necessity of obtaining spontaneous ignition of the injected fuel, whereas the upper limit depends upon the resistance of the materials of which the motor cylinder and the motor piston are made. The lower limit of the maximum pressures of compression for a machine the thermal state of which is the normal operating state, ranges for instance from 25 to 35 atmospheres, whereas the upper limit, in a free piston machine the motor portion of which is made of materials as commonly used at the preesnt time ranges from 80 to atmospheres. The pressures of compression close to the lower limit are to be produced in the motor cylinder for minimum loads applied to the plant or when the plant is running on no load, whereas, for maximum loads, the final compression pressure in the motor cylinder is to come close to said maximum limits.

Obviously I may act on various factors for obtaining a variation of the return energy capable of producing said variation of the final compression pressure in the motor cylinder. However, it seems particularly simple to make use, as a factor of this kind, of the output pressure of the auto-generator (or a pressure corresponding thereto) and to arrange in such a manner the stabilizer which controls the operation of the return energy accumulator, and is operated by said pressure, that this stabilizer causes the final pressure of compression in the motor cylinder to increase when the output pressure increases and to decrease when the output pressure decreases. Instead of directly utilizing the output pressure, i. e., the pressure at the exhaust of the auto-generator, I may utilize an analogous pressure which is practically equal to this output pressure or the variations of which are in relation to the variation of this pressure according to a predetermined law. A pressure which is practically equal to the output pressure at the exhaust of the autogenerator is the pressure of the air present in casing 10,

so that this last mentioned pressure can be substituted as a factor of adjustment, for the output pressure.

In the diagram of Fig. 3, curve A represents the variation, according to my invention, of the final compression pressure in the motor cylinder, as a function of the pressure 2 existing in casing 10. It will be seen that the compression pressure is, for instance, equal to 30 atmospheres for the minimum pressure )1 in the casing, during normal operation of the machine, for the minimum loads of the plant or when said plant is running on no load, whereas the final compression pressure reaches a value of 70 atmospheres for the maximum pressure (Pmax) in the casing, which corresponds to the maximum load on the plant.

When the return energy accumulator is a pneumatic accumulator, in order to obtain the above mentioned variations of the final compression pressure, I vary the mean pressure existing in this accumulator, during normal operation of the machine and between pressures p1 and pmax along the line B of the diagram of Fig. 2. This variation is obtained by varying the mass of air imprisoned in this accumulator. In order to increase the mean pressure and, consequently, the energy that is stored up, I introduce a certain amount of air into said accumulator, whereas, in order to reduce the mean pressure and, con sequently, this energy, I withdraw a certain amount of air. For this purpose, I make use of a stabilizer which works advantageously both as a function of the mean pressure existing in the accumulator or of an analogous pressure and as a function of the pressure, in particular of the mean pressure, existing in casing or of an analogous pressure.

Of course, in order to increase the mass of air in the accumulator, I might introduce air into this accumulator from a reservoir at relatively high pressure and, in order to reduce the amount of air in the accumulator, I might cause a certain amount of air to escape from this accumulator into the atmosphere.

However, it seems preferable to make use of the same reservoir having a pressure intermediate between the maximum and minimum pressures of the pneumatic accumulator, both as a source of air under pressure when it is desired to feed air under pressure into the accumulator and as an air receiver, when a certain amount of air is to be evacuated from the accumulator, this reservoir being preferably constituted by the casing 10 of the auto-generator.

Thus, said casing 10 is connected through a conduit 11 with the outer compartment of cylinder 4, which compartment constitutes the pneumatic energy accumulator and the flow of air into or out of said compartment is controlled by a slide valve 12 operated in response to the mean pressures existing, on the one hand in the energy accumulator and, on the other hand, in casing 10. For this purpose, said slide valve 12 is connected with a piston 13 operating in a cylinder 14 and dividing this cylinder into two chambers one of which communicates, through a calibrated orifice 15, with the inside of the pneumatic energy accumulator, whereas the other one communicates, through a calibrated conduit 16, with the inside of conduit 11 and, consequently, of casing 10. Thus the two faces of piston 13 are subjected to the actions of these two mean pressures. Furthermore, this piston 13 is subjected to the action of a return spring 17 having a tendency to bring or to keep slide valve 12 in a position for which feed air can enter the accumulator but for which air cannot escape from the accumulator.

In order to enable slide valve 12 to produce, at the desired time, either the inflow of a certain amount of air into the accumulator or the outflow of a certain amount of air from the accumulator, said slide valve is given the shape of a box which is divided by a partition 18 into two chambers 19 and 20 and, according to another feature of the invention, said partition is fitted with a check valve or a series of such valves 21 opening all in the same direction.

If these valves 21 are disposed as shown by the drawisng, iil(l)1id can flow only from chamber 19 toward cham- Furthermore, I provide in the lateral walls of each chamber, ports 22 and 23 cooperating with the inner wall of a guiding sleeve 24 in which slide valve 12 moves gxialliyland the inside of which communicates with con- The position of apertures 22 and 23 and the length of guiding sleeve 24 are chosen in such manner that, when slide valve 12 is moved away from its neutral position (as shown by the drawing) for which ports 22 and 23 are closed by the wall of sleeve 24, one of the series of ports is brought into communication with conduit 11, whereas the other series of ports is brought into communication with the inside of the return energy accumulator. If, for instance, due to a movement of slide valve 12 in the upward direction, ports 23 are brought into communication with conduit 11 and ports 22 with the inside of the accumulator, only an inflow of air under pressure into the accumulator is possible, this air then passing from the compartment including ports 23 and past check valves 21 into the compartment including ports 22 and thence, through these last mentioned ports, into the accumulator.

On the contrary, if slide valve 12 is moved in the downward direction, so that ports 22 are brought into communication with conduit 11 and ports 23 with the inside of the accumulator, only evacuation of a certain amount of air from this accumulator is possible. The evacuated air then passes through ports 23, check valves 21, ports 22, to conduit 11.

In order to obtain an operation of said stabilizer corresponding to line B of Fig. 2, it is necessary suitably to choose the ratio of the annular area S of piston 13 subjected to the action of the mean pressure existing in casing 10 to the annular area S of this same piston sub jected to the action of the mean pressure existing in the accumulator. The higher the ratio fi the greater the slope of line B (Fig. 2). The force of spring 17 determines the point where line B intersects the vertical line corresponding to the value p0 of the output pressure.

It should be noted that line B, when the mean pressures in the accumulator and the casing are caused to act directly upon slide valve 12, is in the form of a straight line. However, it is clear that line B (Fig. 2) and, consequently, curve A (Fig. 3) can be given any desired shape by inserting, in a known manner, between one of the parts which are directly under the action of at least one of said mean pressures and slide valve 12, a kinematic device such as a cam which makes it possible to impose upon the displacement of slide valve 12 any desired law as a function of the control pressure which acts through this cam.

The pressure variations such as result from lines A and B (Figs. 3 and 2) are quite suitable at least during the period of normal operation of the machine, that is to say when the machine, once its normal thermal state has been reached, works between its minimum load (running on no load) for which the output pressure is equal to p1 and its maximum load corresponding to pressure Pmax. However, during the starting period, when the machine is still cold, it may be advantageous to increase the final pressure of motor compression and, for this purpose, the mean pressure in the accumulator, beyond the values indicated by lines A and B, within the zone extending between pressure p0 and said pressure p1.

For this purpose, according to still another feature of the invention which may be used separately, I reduce, during the period of starting of the engine, the force against which spring 17 operates, for instance by reducing the pressure, corresponding to the pressure in the accumulator, which acts on the area S of piston 13. I thus transform the initial portions of lines A and B so as to give these portions the shape indicated by the dotted lines of Figs. 3 and 2 and designated by A and B'. The final motor compression pressures indicated by the portion A of curve A ensure starting without risk of breakdown.

In Fig. 4 I have shown a stabilizer capable of ensuring, during the starting period, that is to say between pressures p0 and pi, an operation according to lines B and A and, during the period of normal operation of the auto-generator, between pressures p1 and pmax, an operation according to lines B and A. For this purpose, and according to a first embodiment, I interpose, between a calibrated orifice 15a provided in the wall of the accumulator and the portion of cylinder 14 located above piston 13, a valve 25 provided with a spring 26 which urges it toward its closed position through a piston 27 movable in a cylinder 28. The end 29 of the stem 30 of this valve is applied against the end of piston 27 by a spring 31.

Furthermore, the portion of cylinder 14 located below piston 13 is connected through a conduit 32 with the portion of cylinder 28 located above piston 27.

At the beginning of the starting period, when the pressure in casing is equal to p0, spring 26 acts with the whole of its force on valve 25. The mean pressure of the accumulator transmitted through calibrated orifice a is therefore very much reduced by the pressure drop across said valve 25. Consequently, the pressure that acts upon the upper face S of piston 13 is lower than the mean pressure of the accumulator which, normally, would act upon this surface. This enables spring 17 to keep slide valve 12 in a position for which it establishes communication from casing 10 toward the energy accumulator, but cuts off the communication in the opposed direction. At the beginning of the starting period, when the pressure in casing 10 is p0, the mean pressure in the accumulator is thus higher, by Ap, than the normal mean pressure indicated by the intersection of line B with the vertical line corresponding to pressure 120. A corresponding increase indicated by the intersection of curve A with the same vertical line is obtained for the final motor compression pressure.

As the pressure rises in casing 10, the force with which spring 26 acts upon valve decreases as a consequence of the transmission, through conduits 11, 16 and 32 of the output pressure which acts upon piston 27 in a direction opposed to that of spring 26. When the pressure in the casing reaches value p1, this pressure fully balances the force of spring 26 and valve 25 opens fully, thus ceasing to have a throttling action upon the transmission of the mean pressure in the accumulator to the upper compartment of cylinder 14. The whole of the mean pressure existing in the accumulator then acts upon piston 13 and the operation of the stabilizer becomes that indicated by curves B and A.

According to a modification also intended to transform, between pressures po and p1, lines A and B into lines A and B, which modification is shown by Fig. 5, I provide, between the return energy accumulator and the portion of cylinder 14 located above piston 13, two calibrated channels 15b, 150, these channels replacing the communication afforded, in the preceding embodiment, by passage 15a and valve 25.

Calibrated channel 150 is given a free section much smaller than the free section of calibrated channel 15b and this last mentioned channel opens into cylinder 14 at a place located at a distance from the end wall of said cylinder, in which end channel 150 is provided. Due to this arrangement of channel 15b, the opening of this channel into cylinder 14 is closed by piston 13 at the beginning of the starting period, in view of the fact that, at this time, spring 17 pushes piston 13 upwardly against said end wall of cylinder 14, slide valve 12 being thus in a position for which starting air cannot escape toward casing 10. Calibrated channel 15c, due to its small section, as long as it constitutes the only communication between cylinder 14 and the energy accumulator, delays the building up, in cylinder 14 abovmpiston 13, of a pressure equal to the mean pressure in the accumulator. The stabilizer therefore works, during the starting period, in such a manner as to ensure the operation indicated by curves A and B of Figs. 2 and 3. However, once piston 13 has been pushed by the pressure acting upon its upper face S up to a goint wher'e' it clears the opening of channel 15b into cylinder 14, the pressure acting on surface S becomes equal to the mean pressure in the accumulator and the stabilizer starts working normally so as to ensure an operation according to curves B and A of Figs. 2 and 3.

Of course, I might obtain an operation of the stabilizer analogous to that above indicated, by reinforcing, during the starting period, the pressure that acts upon the under face S of piston 13 instead of reducing, during said period, the pressure that acts upon the upper face S of this same piston.

The main advantages of my invention are, in particular, the rapidity with which the operation of the machine adapts itself to every variation of the load, the simplicity of its construction and the reliability of its operation, in particular the safety with which starting of the machine is obtained.

In a general manner, while I have, in the above description, disclosed what I deem to be practical and efficient embodiments of my invention, it should be well understood that I do not wish to be limited thereto as there might be changes made in the arrangement, disposition and form of the parts without departing from the principle of the present invention as comprehended within the scope of the accompanying claims.

' What I claim is:

1. In combination, a free piston machine including a motor cylinder and a compressor cylinder fixed with respect to each other, a motor piston and a compressor piston rigid with each other and freely movable in said two cylinders respectively, and pneumatic accumulating means for recuperating energy from the operation of said motor piston and cylinder to ensure the return stroke of said motor piston, stabilizer means, differentially operative by a pressure force proportional to the working pressure of the compressor piston and cylinder unit and a pressure force proportional to the mean pressure in said accumulating means, respectively, for controlling the amount of air in said accumulating means, and means for reducing, during the starting period of the engine, the ratio of the first mentioned pressure force to the second mentioned one.

2. In combination, a free piston auto-generator including a motor cylinder and a compressor cylinder fixed with respect to each other, a motor piston and a compressor piston rigid with each other and freely movable in said two cylinders respectively, a casing surrounding said motor cylinder, said motor cylinder being provided with at least one port adapted to be controlled by said motor piston for connecting said casing with the inside of said motor cylinder, the working chamber of said compressor cylinder being on the same side of said compressor piston as said motor cylinder, check valve means opening toward said casing for connecting said compressor working chamber with said casing, and pneumatic energy accumulating means constituted by the chamber of said compressor cylinder located on the other side of said compressor piston from said working chamber, stabilizer means, differentially operative by therespective actions of the working pressure in said casing and of the mean pressure in said accumulating means, for controlling the amount of air in said accumulating means in accordance with the pressure in said casing, and throttle valve means responsive to variations of the working pressure of the compressor piston and cylinder unit for reducing, during the starting period of the machine, the effect of the pressure in said casing with respect to that of the mean pressure in said accumulating means.

3. In combination, a free piston auto-generator including a motor cylinder and a compressor cylinder fixed with respect to each other, a motor piston and a compressor piston rigid with each other and freely movable in said two cylinders respectively, a casing surrounding said motor cylinder, said motor cylinder being provided with at least one port adapted to be controlled by said motor piston for connecting said casing with the inside of said motor cylinder, the working chamber of said compressor cylinder being on the same side of said compressor piston as said motor cylinder, check valve means opening toward said casing for connecting said compressor working chamber with said casing, and pneumatic energy accumulating means constituted by the chamber of said compressor cylinder located on the other side of said compressor piston from said working chamber, a hollow box, a partition dividing the inside of said box into two compartments, check valve means in said partition opening toward one of said compartments, and throttle valve means responsive to variations of the working pressure of the compressor piston and cylinder unit, differentially operative by the respective actions of the working pressure in said casing and of the mean pressure in said energy accumulating chamber, for connecting the above mentioned compartment with said energy accumulating chamber and the other compartment with said casing in response to a relative rise of the pressure in said casing and connecting the first above mentioned compartment with said casing and the other compartment with said energy accumulating chamber in response to a relative drop of the pressure in said casing.

4. In combination, a free piston machine including a motor cylinder and a compressor cylinder fixed with respect to each other, a motor piston and a compressor piston rigid with each other and freely movable in said two cylinders respectively and pneumatic accumulating means for recuperating energy from the operation of said motor piston and cylinder to ensure the return stroke of said piston, a reservoir containing air at a pressure intermediate between the maximum and minimum pressures in said pneumatic accumulating means, a stabilizer device for placing the inside of said pneumatic means in communication with said reservoir, said device including a sliding part having an intermediate position for which said communication is cut oil and movable in opposite directions from said position to open said communication and check valve means for permitting said communication, for one of said directions of displacement of said sliding part, only from said reservoir toward said pneumatic means, whereas said communication is only from pneumatic means toward said reservoir for the other direction of displacement of said sliding part, a cylinder, and a piston movable in said last mentioned cylinder rigid with said sliding part for actuating it, one end of said last mentioned cylinder being in communication with said reservoir so that the pressure in said reservoir urges said last mentioned piston and said sliding part in the first mentioned direction, and the other end of said last mentioned cylinder being in communication with the inside of said pneumatic means so that the pressure in said pneumatic means urges said last mentioned piston and said sliding part toward said second mentioned direction, the area of the face of said last mentioned piston turned toward the first mentioned end of said last mentioned cylinder being smaller than the area of the face of said last mentioned piston turned toward the second mentioned end of said last mentioned cylinder.

References Cited in the file of this patent UNITED STATES PATENTS 2,007,305 Pescara July 9, 1935 2,178,310 Pescara Oct. 31, 1939 2,408,089 Muntz Sept. 24, 1946 2,423,720 Mullejans et a1. July 8, 1947 2,463,051 Pescara Mar. 1, 1949 FOREIGN PATENTS 430,042 Great Britain June 12, 1935 438,418 Great Britain Nov. 15, 1935 568,382 Great Britain Apr. 3, 1945 866,568 France 1941 

